This disclosure generally relates to processes for making composites comprising at least one metal or metal alloy and an electrolyte associated with a support, and more specifically processes suitable for making an aerogel-electrolyte-metal composite.
Supported metals find utility in a wide range of applications, for example as electrodes and/or catalysts. Known supports include aerogels, which are porous materials that may be produced by sol-gel polycondensation reactions. Many aerogels have small inter-connected pores. The chemical composition, microstructure, and physical properties of aerogels may be controlled on the nanometer scale by adjusting the sol-gel processing conditions.
Electrodes have been made from monolithic, granular and microspherical carbon aerogels (also referred to as carbon foams). Such electrodes are used, for example, in energy storage devices such as capacitors and batteries, in fuel cells, e.g., proton exchange membrane (PEM) fuel cells, in electrocapacitive deionization devices, and the like. Fuel cells are believed to be a promising alternative to present power sources because fuel cells are able to convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. Among various types of fuel cells, PEM fuel cells are particularly attractive because they operate at low temperatures, allow immediate response to changes in the demand for power, and exhibit high efficiencies and power densities.
PEM fuel cell systems are based on a membrane electrode assembly (MEA), which includes a polymer electrolyte (the “membrane”) disposed between an anode and a cathode. Hydrogen is channeled to the anode at which it forms hydrogen ions (protons) and electrons in the presence of a metal catalyst. The polymer electrolyte membrane allows only the positively charged ions to pass through it to the cathode. The electrons must travel along an external circuit to the cathode, creating an electrical current. At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell.
In PEM fuel cells, precious metals such as platinum are the most active catalysts. Unfortunately, platinum is expensive and electrodes are frequently limited by low platinum utilization (i.e., percentage of platinum that can be accessed by reactants for catalysis). Some applications may tolerate increased platinum concentrations to compensate for the low utilization level. However, in order for PEM fuel cells to be more commercially viable, a reduction in the cost per kilowatt (kW) is desirable. This can be achieved by minimizing the precious metal concentration and/or increasing the precious metal utilization in the electrode.
One approach uses platinum crystals supported on a high surface area carbon, such as an aerogel, rather than pure platinum black crystallites as the catalyst. As used herein, the term “crystals” includes crystalline-like structures, partially crystalline structures and agglomerations of crystals, partially crystalline structures, and/or amorphous or semi-amorphous particles. One difficulty with existing processes for making aerogels comprising metalcrystals is the inadequate control over the manner in which metalcrystals are incorporated. Frequently, composites with inconsistent metal crystal sizes, broad crystal size distributions and poor electrolyte-to-catalyst contact are formed. This has been one of the factors impeding the commercialization of aerogel supported catalysts and/or electrodes, particularly for use as a MEA in a PEM fuel cell.
There accordingly remains a need in the art for improved aerogel-metal composites, particularly carbon aerogel-metal composites, and improved processes for making such composites. It would be particularly beneficial if such composites and processes resulted in decreased metal crystal concentration and/or improved exposure of catalysts to the reactants at the electrolyte/catalyst interfacial region for improved performance and/or reduced cost of fuel cells and other devices using supported electrocatalysts.